Paper machine clothing comprises circulating belts of considerable length and width in paper machines, and is used for forming and guiding the paper web through the paper machine. Its construction thereof strongly depends on the parts of the paper machine in which it is used. In the first two parts of a paper machine, the sheet forming section and the press section, the paper machine clothing has to be designed so that the paper web is dewatered as thoroughly as possible, so that the energy consumption for thermal drying in the subsequent drying section remains low. Dewatering occurs through the paper machine clothing, i.e. the latter has to be constructed so that the liquid coming from the paper web is transported as efficiently as possible to the other side of the paper machine clothing.
In order to ensure this, very different designs of paper machine clothing have been developed. Forming screens, which generally consist of a woven fabric as a support, are provided for the sheet forming section. Pure woven fabrics such as these have also already been proposed for the press section. However, under the high pressure in the presses of the press section they tend to form marks in the paper web, which is why what are termed press felts are commonly used, as before, in which a support is embedded in a fibrous matrix. In particular, the support serves to withstand the tensile forces acting on the paper machine clothing, and is principally formed as a woven fabric, wherein the support can also be formed from a plurality of supporting lengths which are not joined to each other. A fibrous web, which protects the support from abrasion and which ensures flat surfaces, is then needle-bonded to one or both sides of the support.
Instead of a woven fabric consisting of longitudinal and transverse filaments, other textile filament constructions can also be used for the support, such as knitted fabrics, fibre lay-ups or fibre assemblages (see EP-B-0 394 293, EP-A-0 038 276). Different types of formed products consisting of filaments can also be combined with each other, as is shown, for example from DE-C 40—40 861. In addition, supports consisting of meshes are also known (see WO 92/17643; EP-B-0 307 182).
The filaments of which textile supports consist generally have a circular cross-section. Filaments of flat, rectangular cross-section are also used for certain purposes. In addition, there is a series of proposals which have been made and which relate to other cross-sectional shapes of filaments for the supports of paper machine clothing or for the fibres of nonwoven supports, some of which are strongly profiled (DE-A-40 31 608; U.S. Pat. No. 5,361,808; DE-A-195 45 386; U.S. Pat. No. 5,591,525; EP-A-1 067 239; DEA-199 00 989; EP-A-0 995 835). These filaments or fibres are profiled for very different reasons. A common feature of all filaments or fibres is that the profile extends longitudinally, i.e. it extends along the axis of the respective filament.
An important object for press felts in particular is to fashion their structure so that dewatering occurs as effectively as possible. For this purpose, it is important that the paper machine clothing has a good liquid absorption capacity and a low resistance to flow. Efforts are therefore made, whilst taking other requirements such as strength, freedom from marks and the like into consideration, to provide an open volume which is as high as possible inside the paper machine clothing.
In order to achieve this object, it is proposed in GB-A-1 053 282 and U.S. Pat. No. 3,158,984 that profiled monofilaments of very different cross-sectional shapes are used, which are twisted helically so that helical profiling results. The monofilaments are twisted independently of each other about their longitudinal axis and are fixed in this state inside the support.
On account of the individual twisting of the monofilaments, the production of a support such as this is complicated, which is why paper machine clothing such as this has not found widespread use. Moreover, it is very difficult to achieve uniform twisting of the monofilaments and thus to achieve uniform dewatering properties over the surface. Furthermore, monofilaments tend to untwist depending on the inherent stress, with corresponding effects on the dewatering properties.
The underlying object of the present invention is to fashion paper machine clothing of the type cited at the outset so that good dewatering properties are achieved by a simple manufacturing process.